1. Field of the Invention
The present invention relates to a low-quantum-defect optical amplifier and a laser including such an amplifier.
2. Description of the Related Art
Placed inside a resonant optical cavity, an optical amplifier makes it possible to realize a laser and such to produce a coherent light flux.
Moreover, conventionally, to produce powerful laser beams, amplifiers are used. To amplify a laser beam, at a determined emission wavelength, an optical pump is used whose wavelength is lower than that of the emission wave and which, according to the prior art, is relatively far from the emission wavelength.
It is generally considered that the amplification gain is maximum for pump and emission wavelengths that are relatively far from each other. From then on, simple dichroic filters can be used to separate the radiations at wavelengths near to the pump wavelength. They allow coupling of the pump light flux in the amplifying crystal and extraction of the amplified useful flux.
The quantum defect ρ is defined as the parameter that represents the difference between laser emission wavelength (λL) and pump wavelength (λP) of an optical amplifier:
  ρ  =      1    -                  λ        P                    λ        L            
The today high-performance amplifiers have a quantum defect higher than a minimum value of the order of 10%.
Various crystals may be used as an amplifying medium. In particular, ytterbium-ion-doped CaF2 crystals have sometimes been used, at ambient temperature, in laser systems operating in femtosecond regime. Still in this case, the pump wavelength is relatively far from the emission wavelength.
In this context of high-power laser beam production, the document U.S. Pat. No. 6,195,372 describes various cryogenic-temperature cooling devices for YAG-rod solid lasers. According to this document, the cryogenic cooling, i.e. near to 100-150 K, of a YAG-rod laser allows simultaneously increasing the thermal conductivity and reducing the thermal expansion coefficient of the material, which leads to an increase of the laser mean power.